This application claims priority from Japanese Patent Application No. 316251/1992 filed Oct. 30, 1992, which is incorporated herein by reference.
A semiconductor laser module is a small, multipurpose light source comprising a semiconductor laser, a lens and a receptacle into which a ferrule holding an end of an optical fiber can be inserted or from which it can be plucked easily. In general, a semiconductor laser module has a wide scope of utility for a light source of optoelectronic communication or measurements using optical fibers for transmitting media. In a semiconductor laser module, laser beams are converged on an end of an optical fiber by a lens. Most of the beams enter the optical fiber. But some part of the beams is reflected at the end of the optical fiber. The light reflected at the front end of an optical fiber is called near end reflection light. Since a semiconductor laser synchronously amplifies light power by reflecting light repeatedly by resonator mirrors at both ends of the chip, returning of the near end reflection light will cause instability in the oscillation action of the laser. The instability due to the return of the reflection light is an inherent weak point of a semiconductor laser which depends on induced emission of light. Various improvements have been devised in order to prevent reflection light from returning to the laser.
An easy solution is to shear an end of the fiber obliquely in order to keep reflection light from returning to the laser. FIG. 7 exhibits a prior pig tail type module with an improvement of forbidding reflection light returning into the laser. The module has a semiconductor laser (1), a lens (2), a ferrule (4) holding an end of an optical fiber (3) and a ferrule holder which fixes them everlastingly. Namely, the ferrule (4) can not separate from the holder in usual use. The near end reflection light does not, return to the semiconductor laser (1), because the front end of the fiber has been obliquely sheared and the reflection light deviates from the laser at twice the oblique angle of the end. Although the module is simple from the standpoint of optics, it is rather complex from the standpoint of assembly or construction. In general, an end of an optical fiber is cut and polished perpendicular to the axial line. Such a perpendicular cut fiber cannot be directly connected to a semiconductor. Therefore, the module has another connector besides the ferrule holder keeping the semiconductor laser, lens, and the ferrule. The connector terminates the other end of the optical fiber whose front end is fixed by the ferrule and front surface has been obliquely sheared. Any optical fiber having a ferrule can freely be coupled or decoupled to the rear connector. The optical fiber for transmitting optical signals has a perpendicularly cut end. The module requires an intermediate fiber (3) between the ferrule holder and the extra connector. Since the intermediate fiber looks like a tail of a pig, this type module is called "a pig tail type".
A pig tail module needs the intermediate fiber for coupling the laser and an optical fiber. As it includes double connections, material costs and assembly costs have piled up because of an excess of parts, e.g. an extra connector and a ferrule.
Some improvements have been proposed to avoid such double connections of pig tail type. One is a module to which a ferrule having a perpendicular cut fiber end can be attached or removed directly. It dispenses with an intermediate fiber, a connector and a ferrule. The ferrule of the external fiber can be directly inserted into a hole of a receptacle. Since the front end of the fiber has been cut perpendicular to the axial line of the fiber, the reflection from the front surface of the fiber end goes back to the semiconductor laser along the axial line. Two kinds of devices have been proposed in order to avoid returning of the reflection to the laser.
One improvement is to provide a key-shaped transparent block in front of a semiconductor laser. The other improvement is to incline a beam axis from the central optical axis. The prior improvements will be explained in more detail.
[1. key-shaped transparent block]
Japanese Patent Laying Open No.2-81008 (81008/1990) has proposed an improvement by a key-shaped transparent block for prohibiting the reflection light from returning to a semiconductor laser. FIG. 3 shows a schematic view. FIG. 5 exhibits a concrete module. The light beams emitted from a semiconductor laser (1) are converged by an lens (2) to a front end of a optical fiber (3). A key-shaped glass block (5) is disposed in front of a ferrule (4) holding the end of the fiber (3). In practice, the key-shaped glass block (5) is fixed at a front surface of a receptacle (6) with an axial hole for keeping the ferrule (4). The ferrule (4) can be put into the axial hole and taken off from the hole. In the state that the axial hole is pierced by the ferrule, the front end of a fiber is in contact with the key-shaped glass block (5). As the outer surface of the key-shaped glass block inclines to the optical axis of the fiber, the beams from the semiconductor laser (1) are refracted by the block (5) and enter the fiber (3). The incident beams are slightly slanting to the optical axis. But the inclination is less than the aperture angle of the optical fiber. The incident beams can be converted to propagating beams in the fiber (3). The aperture means a light cone in which the beams going out of a fiber end spread. Reversely, any beams within the aperture (light cone) can enter a fiber as propagating beams. The aperture angle is determined by the refractive indices of the core and cladding.
The glass block (5) is in tight contact with the fiber (3). If the refractive index of the glass is equal to that of the fiber, no reflection occurs at the boundary between the glass and the fiber. The tight contact is a necessary condition for no reflection. Of course, other reflection at the outer surface of the glass block (5) does occur as shown by a dotted line in FIG. 3. However, this reflection does not return to the semiconductor laser, because the reflection deviates from the optical axis between the laser and the fiber. Thus, the reflection from the outer surface of the block brings about no bad influence upon the stability of the laser oscillation.
The function has been clarified. The practical structure of the improvement will be explained by FIG. 5. In the figure, a semiconductor laser chip (1) fitted on a mount is encapsulated in a semiconductor laser package (9). A spherical lens (2) is fixed in an axial bore perforated through a lens holder (8). The semiconductor laser package (9) is equipped to a front end of the axial bore of the lens holder (8). A receptacle (6) has an axial hole and a flange. A ferrule with a fiber end can be attached in the axial hole and removed from it. The fiber end has been cut and polished perpendicular to the optical axis. In some cases, it may be polished round. The axial hole is stepwise widened near the flange. A step (18) is formed at the transition of the diameter of the hole. A key-shaped glass block (5) is held by a cylindrical stopper (7). The stopper (7) is inserted into the larger hole from the flange side. Then, the stopper (7) contacts with the step (18). The stopper (7) is fixed there. One end of the glass block is obliquely polished. The other end of the glass block is perpendicularly polished in order to couple to a fiber end without a gap therebetween. At the ferrule side, the surfaces of the stopper (7) and the block (5) are coincident with each other in a plane. When the ferrule keeping the end of a fiber is inserted into the axial hole, the front end of the fiber comes into tight contact with the glass block (5). The lens holder (8) is welded to the flange side of the receptacle (6).
[2. inclining beam axis]
For example, Japanese Patent Laying Open No. 3-45913 (45913/1991) has proposed an improvement utilizing an inclining beam axis. FIG. 4 shows the principle of the improvement. FIG. 6 demonstrates a concrete module. The beam line drawn from a semiconductor laser (1) to a center of a fiber end via a lens (2) inclines to the optical axis of the fiber (3). Namely, the laser (1) and the lens (2) are not aligned along an extension of the axial line of the fiber (3). The beams obliquely enter the end of the fiber within the aperture cone. Some part of the beams is reflected at the end surface as shown by a dotted line in FIG. 4. But the reflection does not return to the laser (1), because the incident angle is not a right angle. In FIG. 4, the laser (1), lens (2) and the center of the fiber end lie on a straight line (m). Line (m) is not the same as fiber axial line (n). Line (m) and line (n) cross each other at the center of the fiber end.
Namely, a first optical axis (m) determined by the laser (1) and lens (2) is slightly inclined to a second optical axis (n) determined by the fiber (3). The inclination of axes is important. The inclination prohibits the near end reflection light from returning to the semiconductor laser.
The near end of the fiber is protected by a ferrule (4) having a cylindrical shape. The ferrule can be put into an axial hole of a receptacle (6) and can be removed from the axial hole. The end of the fiber has been polished flat or slightly round perpendicular to the axial line. Light beams emitted from the laser (1) are converged by the lens (2) and enter the fiber (3). The inclination angle between line (m) and line (n) is less than the aperture angle. The beams entering the fiber can be converted into a propagating mode in the fiber. The incident beams may be slightly weaker than the perpendicular incidence beams.
What is important is where the reflection light goes. Since optical axes ( m ) and ( n ) are slightly slanted, the reflection light does not return to the laser (1). Practical module is demonstrated by FIG. 6. A lens (2) is fixed in an axial bore of a lens holder (8). A receptacle (6) has a flange and an axial hole with a step (18). A cylindrical stopper (7) is fixed in the axial hole of the receptacle (6). When a ferrule is inserted into the axial hole, the stopper (7) determines the position of the ferrule. The laser chip (1) is airtightly sealed by a semiconductor laser package (9). The lens holder (8) is welded at a pertinent eccentric position on the flange of the receptacle (6). The laser package (9) is adjusted in order to accomplish an optimum coupling to a fiber (3) and is welded at the position on the end surface of the lens holder (8).
This improvement can feature a crossing of optical axes. But Japanese Patent Laying Open No. 3-45913 has defined the improvement by a reverse eccentricity of optical axes of laser and receptacle with regard to the axis of lens. The beams converged by the lens enter the fiber slantingly to the fiber axis. The near end reflection light goes back to other direction deviated from the laser.
Prior laser modules for solving the difficulty of the near end reflection light have been explained so far. The pig tail type which forbids the reflection returning to a laser by oblique shearing of fiber end has the drawbacks of manifold parts and high assembly costs. Two improvements which had devised to conquer the drawbacks also have disadvantages.
[1. disadvantage of the key-shaped transparent block type]
Such an improved module with a key-shaped transparent block has a difficulty of a growing gap between a block and a fiber end. If attaching and removing of ferrules are repeated many times, the front end of ferrules does not come in tight contact with the glass block. Some clearance occurs between them. The clearance incurs an increase of reflection noises and an abasement of modulation performance of signals. The tight contact of a ferrule with a glass block is an important requirement for removing reflection at the surfaces of a fiber and block as mentioned before. The modules of ill-function were investigated by the Inventors for seeking the reason of malfunction. The Inventors discovered that some foreign matter adhered to the glass block. The foreign matter was sandwiched by the ferrule and block. The tight contact of the ferrule with the glass block was disturbed by the foreign matter. Then, the reflection at the surfaces increased. Analysis taught the Inventors that the foreign matter was the material of receptacle or cotton flakes for cleaning ferrules or receptacles.
The foreign matter could not easily be removed, because it adheres to the glass block by the strong force applied on ferrules. Since semiconductor laser modules are used in optoelectronic communication system as industrial products, it is inevitable that frequent repetitions of putting on or taking off the ferrule would induce abrasion of a receptacle and intake of dust by ferrules. The glass block is placed at the bottom of the axial hole of a receptacle. It is difficult to get rid of the foreign matter from the surface of the glass block.
Thus, the improvement of the key-shaped transparent type has a drawback that it is difficult to keep a tight contact between the ferrule and block which is indispensable for ensuring the reliability of the module owing to adhesion of dust to the glass block.
[2. disadvantage of the inclining optical axis]
The improvement of the inclining optical axis has also a drawback that it is difficult to adjust the relative position of a laser, lens and ferrule, because they must align on a line inclining to the central axis of the lens holder or the receptacle. Namely, the positions of the laser must be eccentric to the axial bore of the lens holder. The receptacle must be eccentric to the lens holder. FIG. 4 and FIG. 6 show eccentric dispositions of the receptacle (6) to the lens holder (8) and of the laser package (9) to the lens holder (8). The laser (1) and the receptacle (6) deviate from reciprocal direction with regard to the optical axis of the bore of the lens holder (8). The eccentricity decides the incident angle of beams to the optical fiber (3). In this case, the incident angle is not 0 degree unlike in ordinary modules. If the incident angle to the fiber were too large, the coupling loss between the fiber and the laser would Increase. On the contrary, if the incident angle were too small, the near end reflection light would return to the laser and cause instability to the laser oscillation. Therefore, the incident angle of beams to a fiber must be rigorously determined at a definite angle which is neither too large nor too small. The eccentricity must be rigorously decided with precision.
Experiments taught the Inventors that .+-.50 .mu.m of error of deviation would induce .+-.1 dB of fluctuation of the coupling efficiency between a laser and a fiber under the conditions of 3 mm of the distance between a fiber and a lens and 5 degrees of the slanting incident angle. The conditions that the distance is 3 mm and the incident angle is 5 degrees are general in this type modules. But .+-.1 dB of the coupling efficiency is not so small fluctuation as can be neglected.
In an actual assembly, it is difficult to fix a lens holder eccentric to a receptacle and a laser package eccentric to a lens holder within tens of micrometers of tolerance. The assembly would invite incompatibility among assembly time, yield, and performance. Namely, it would take a long time to assemble the modules of high performance with high yield. Otherwise, if we tried to assemble the products in a short time, the performance would degenerate.
Furthermore, in the modules of inclining axis type, the coupling between a receptacle and a lens holder will incur asymmetric region of welding. In FIG. 6, the upper side of a welding region is much wider than that of the lower side. Such imbalance will deeply impair the strength of welding. Thus, the defaults of the inclining optical axis type are the difficulty of assembly and the mechanical fragility.
One purpose of this invention is to provide a semiconductor laser module immune from the defaults of prior ones. Namely, the semiconductor laser module of this invention is easy to assemble. The mechanical strength is sufficient. The fluctuation of performance is little. Frequent repetitions of putting on and taking off ferrules incur no decline in performance. In short, the laser module of this invention excels in reliability and cost.